1. Field of the Invention
The present invention relates to a charging method which is capable of primarily charging the surface of a photosensitive material without relying upon the corona discharge.
2. Description of the Prior Art
According to an image-forming method based upon the electrophotographic method, an image is formed by uniformly charging the surface of a photosensitive material, exposing the surface of the photosensitive material to the image-bearing light to form an electrostatic latent image that corresponds to the image of the document, and developing and transferring the thus formed electrostatic latent image.
In the above image-forming method, the surface of the photosensitive material has generally been charged (primarily charging) by the corona discharge, generating, however, ozone which is hazardous to the environment. In order to prevent the generation of ozone, in recent years, there have been proposed methods of primarily charging the surface of a photosensitive material by bringing a charging conducting rubber roller into frictional contact with the surface of the photosensitive material while applying a bias voltage (Japanese Laid-Open Patent Publications Nos. 149669/1988 and 267667/1989).
However, the above-mentioned charging methods relying upon the frictional contact have a problem in that uniformity of the electric charge is lost when contaminations such as dust, paper powder, etc. are clogged between the electrically conducting rubber roller and the photosensitive material, making it difficult to stably continue charging. Moreover, in case the surface of the photosensitive material is poorly cleaned and toner is left on the surface, the residual toner adheres on the surface of the photosensitive material at the time of forming image causing the photosensitive material to lose durability. In order to accomplish uniform charging, furthermore, the application of a DC bias voltage only is not sufficient and an AC bias voltage must be applied as well.
In order to solve the above-mentioned problems, the present applicant has previously proposed a method of effecting frictional-charging by using an electrically conducting brush roller to bring a flexible and electrically conducting sheet into frictional contact with the surface of the photosensitive material while applying a DC voltage to the roller (Japanese Patent Application No. 68148/1992).
According to this charging method, the flexible and electrically conducting sheet is depressed by the electrically conducting brush roller and is brought into intimate contact with the surface of the photosensitive material, giving a great advantage in that the frictional-charge can be uniformly carried out by the application of a low DC bias voltage only without the need of applying an AC bias voltage. In case defects such as pinholes exist in the surface of the photosensitive material, however, the above charging method permits the electrically conducting sheet to come in contact with the pinholes. Therefore, a large current flows into the pinholes and the output of the power source drops, resulting in poor charging.